Why a Fridge Makes Cold

First, go and touch it

Really. Go to the kitchen, reach round the back or underneath your fridge, and find the metal — a grille of black pipes, or a flat panel low down.

If the fridge has been running, it is warm. Sometimes properly warm.

Come back. That warmth is about to explain the whole machine, and it is going to do it by proving that the thing you thought a fridge does is not a thing at all.

There is no such thing as cold

This sounds like a trick sentence. It is not.

Heat is real. It is the jiggling of the atoms in a thing — fast jiggling in your tea, slow jiggling in an ice cube. You can add heat to something. You can take heat away. You can watch it move.

Cold is not a second substance that does the opposite. Cold is just less heat. There is no cold to pour, no cold to make, no supply of cold anywhere to run out of. The word is a description, like quiet. Nobody makes quiet. You remove noise.

So when a fridge "makes the inside cold", it cannot be adding anything. There is nothing to add. The only move available is to take heat out of the box — and heat that is taken out has to go somewhere.

You have already felt where.

There is one rule about heat, and it never bends

Heat only ever moves one way on its own: from the warmer thing to the cooler thing.

You already know this without being told. A hot drink goes cold in a cool room; a cool room never warms itself up by draining the drink. An ice lolly melts in your hand — heat goes out of your hand and into the lolly, never the reverse.

That rule is the whole of the fridge's problem. If it wants heat to leave your milk, it has to put something colder than the milk right next to it. Nothing else will do it. Heat will not walk into a warmer place to be helpful.

So the fridge needs one thing: something properly cold, on demand, forever. Where does it get that?

Two things you can check with stuff you already own

Squeeze a gas and it gets warmer. Pump up a bike tyre, then feel the barrel of the pump. It is warm, and not just from your hands. You crammed the air into less room and it heated up.

Let a gas spread out and it gets colder. Hold an aerosol can and spray it for a few seconds. The can goes cold, fast — sometimes cold enough to sting. The gas rushing out into open air cooled sharply as it spread.

That is the whole secret. There is a gas sealed inside the pipes of your fridge, and the humming motor does exactly these two things to it, in two different places:

And now the one rule does all the work by itself.

Follow one bit of warmth out of the milk

Pick one small scrap of heat sitting in a bottle of warm milk you just put in the fridge, and follow it out of the house. Press through and check the rule at every step: heat is always going from the warmer thing to the cooler one.

Warmer than the kitchen, or colder?

One machine, and at any moment parts of it are the hottest thing in the room and parts of it the coldest. Sort each one and read why.

You already own three of these

Once you stop calling it a cold-maker and start calling it a heat-mover, you spot the same machine everywhere.

An air conditioner is a fridge with its warm end hanging out of the window. Cold pipes inside the room, hot pipes outside the building. That is why standing next to the box outside an air-conditioned shop is unpleasant — you are standing in the exit, and the heat coming at you used to be indoors.

A freezer is the same idea pushed further, and a car's air conditioning is the same idea again with the exit at the front of the car.

Here is the test for all of them. Find where the heat comes out. If you cannot find a warm end somewhere, it is not cooling anything — it is only stirring the air around.

What to keep